Exposure device for manufacturing a display screen of a color television picture tube

ABSTRACT

An exposure device for manufacturing a display screen of a color television picture tube. The device comprises a correction lens system provided with a lens having a stepped lens profile of annular zones. Adjoining annular zones are connected by an annular transition region; the surfaces of which are parallel to the light rays to prevent scattering. The correction lens system may perform a translation during the exposure so as to divide any remaining scattered light rays over the entire display screen; the center of the corrective lens system describing a path which is closed upon itself. For example a circle of 1.5 mm.

United States Patent 1 Barten et a1.

[ EXPOSURE DEVICE FOR MANUFACTURING A DISPLAY SCREEN OF A COLOR TELEVISION PICTURE TUBE U.S. Philips Corporation, New York, NY.

Filed: Apr. 24, 1972 Appl. No.: 246,672

Assignee:

[30] Foreign Application Priority Data May 8, 1971 Netherlands 7106346 U.S. Cl 95/1 R, 313/92 B Int. Cl. G03 H0lj 29/18 Field of Search 95/11313/92 B June 12, 1973 3,628,850 12/1971 Yamazaki 95/1 R 3,282,691 11/1966 Morrell et al. 313/92 B Primary Examiner-Samuel S. Matthews Assistant Examiner-Richard M. Sheer Attorney-Frank R. Trifari [5 7] ABSTRACT An exposure device for manufacturing a display screen of a color television picture tube. The device comprises a correction lens system provided with a lens having a stepped lens profile of annular zones. Adjoining annular zones are connected by an annular transition region; the surfaces of which are parallel to the light rays to prevent scattering. The correction lens system may perform a translation during the exposure so as to divide any remaining scattered light rays over the entire display screen; the center of the corrective lens system describing a path which is closed upon itself. For example a circle of 1.5 mm.

2 Claims, 1 Drawing Figure EXPOSURE DEVICE FOR MANUFACTURING A DISPLAY SCREEN OF A COLOR TELEVISION PICTURE TUBE The invention relates to an exposure device for manufacturing a display screen of a colour television picture tube. This device comprises a light source for the optical projection of a pattern of holes in a colour selection mask on a window with a photosensitive layer provided thereon for the manufacture of the display screen. During operation of colour television display tube a number of electron beams scan the colour selection mask under the influence of a deflection system and a convergence and as a result the same number of electron-optical projections of the colour selection mask are formed on the screen. This device furthermore comprising a correction lens system for bringing the optical projection in agreement with an electron optical projection. This correction lens comprising at least one lens having a stepped lens profile which consists of annular zones.

Such a device is known from the US. Pat. No. 3,385,184.

A colour television picture tube manufactured with such a device comprises generally means for producing three electron beams which are eccentric relative to the axis of the tube, a display screen comprising a large number of triplets of phosphor dots, and a colour selection mask at a short distance from and substantially parallel to the display screen. The colour selection mask comprises a large number of holes each corresponding to a triplet. Each triplet comprises generally phosphor dots which luminesce red, green and blue, respectively, when an electron beam impinges on them. Under the influence of a deflection system, the three electron beams collectively scan the colour selection mask in which they cross each other under the influence of a convergence system at the area of the colour selection mask. As a result of the angle which the electron beams enclose with each other, the electrons which pass the colour selection mask through the holes each impinge upon separate phosphor dots. As a result of this it is possible to divide the colour information and the brightness information of the picture to be displayed into the three said colours and to cause each to control separately an electron beam.

Since the pattern of the phosphor dots on the screen must accurately correspond to the pattern of the holes in the colour selection mask, the same colour selection mask is generally used during the manufacture of the display screen, which mask is also present in the completed tube. The patterns of the red, the green and the blue phosphor dots are each provided separately by means of a photosensitive layer which is exposed through the colour selection mask. The colour selection mask is in the same position relative to the display screen and afterwards during operation of the tube. For each colour, the light source which is used for the projection should be given a different place in accordance with the three different deflection points of the electron beams in the deflection system. A deflection point is to be understood to mean the point of intersection of the tangents at the axis of an electron beam not yet deflected and at the axis of a deflected electron beam.

From the above-mentioned United States Patent Specification it is known that the deflection point experiences a displacement as a function of the deflection angle. This displacement consists of an axial displacement as a result of the action of the deflection system and a displacement in the direction of the already mentioned eccentricity of the beam mainly as a result of the action of the convergence system. Axial is to be understood to mean the direction parallel to the axis of the tube normal to the center of the display screen. The said displacement of the deflection point of the electron beams should be imitated with a virtual displacement of the light source dependent upon the deflection angle so that during operation of the tube the pattern of the phosphor dots corresponds accurately with the places of incidence of. the electron beams on the screen. This may be done with a correction lens which produces both the axial displacement and the displacement in the direction of the eccentricity of the beam of the virtual light source. It can be proved that a correction lens which produces the said two displacements of the virtual light source as a function of the deflection angle and which comprises continuous lens surfaces, also produces a displacement at right angles to the axial direction and at right angles to the direction of the eccentricity of the beam of the virtual light source for given aximuthal angles. This displacement is detrimental because it generally does not correspond to a corresponding displacement of the deflection point of the electron beam. The said displacement causes an azimuthal compression of the triplets of phosphor dots on the screen or, which comes to the same thing, a radial expansion. The radial direction is to be understood to mean herein the direction in the plane of the display screen from the center of the display screen to the rele-- vant point on the display screen to which the three beams are deflected. The azimuthal direction is the direction at right angles thereto in the plane of the display screen. As a result of this a poor screen filling can be obtained. It is to be noted that as a result of the curvature of the display screen, a radial compression also occurs so that it is possible in principle to compensate a part of the radial expansion herewith. However, this is possible .only to a restricted extent. In order to check the drawbacks of the said undesirable displacement, the US. Pat. No. 3,385,184 provides a lens having a stepped lens profile. Said lens comprises a number of annular zones. The transition regions between said zones, however, scatter the incident light in undesirable directions. If said transition regions are made opaque, they would be visible as annular zones during the exposure if no further measures were taken, The said Unites States Patent Specification states the use of two lenses having a stepped lens profile to avoid the said difflculties. The lenses are used one by one, each during approximately half of the exposure time and have annular zones the transition regions of which in one lens are staggered relative to those in the other lens. Said transition regions are made opaque in each lens, if desirable with a certain variation in the light transmission. The

use of two lenses and two exposures is a complication in the manufacture and requires an extra exposure device per pattern of phosphor dots to be provided.

It is the object of the invention to avoid this drawback and to provide a lens having a stepped lens profile, the transition regions of which cause substantially no light scattering. It is a further object of the invention to eliminate the influence of any remaining light scattering.

According to the invention, an exposure device of the type mentioned in the preamble is characterized in that adjoining annular zones are connected by an annular transition region the surface of which everywhere touches substantially light rays of the light source in the immediate proximity of the transition region. The invention is based on the recognition of the fact that the light scattering is caused in particular by the fact that the light rays in the known lens intersect the lens surface also in the transition regions. Viewed in the direction of the light rays, the transition regions then become much wider than is necessary. By causing the direction of the light rays to coincide with the surface of the transition region at that area, the width of the transition regions is reduced to substantially zero.

In order to eliminate the influence of any remaining light scattering by the transition regions, the correction lens system in a device according to the invention is preferably arranged so as to be movable for carrying out a translation in which the center of the correction lens system describes a path which is closed in itself. By causing the correction lens system to perform such a translation, the scattered light rays are distributed over the surface of the colour selection mask, as a result of which their influence is negligible since due to the invention very little is scattered as it is. The number of translations is not critical and ten of them are generally sufficient.

The invention will be described in greater detail with reference to the accompanying drawing of an embodiment.

The exposure device shown in the drawingcomprises a housing 1 on which a window 2 is provided of a colour television display tube to be manufactured. A shadow mask 3 comprising a pattern of holes 4 is provided in the window. The window comprises a photosensitive layer 5. The pattern of holes 4 is projected on the layer 5 by means of light from the gas discharge lamp 6. After the exposure, the layer 5 is developed and a pattern of phosphor dots is obtained.

The light of the lamp 6 is concentrated by means of the quartz cone 7 and reaches the layer 5 via a correction lens system 8. The lamp 6, the cone 7 and the correction lens system 8 are aligned along an axis 9 which is eccentric relative to the axis 10 of the device which is at right angles to the center of the window. The eccentricity corresponds to the eccentricity of the means producing the electron beam in the colour television display tube to be manufactured.

The correction lens system 8 comprises two lenses 11 and 12. The lens 11 is a rotationally symmetric lens which produces an increasing axial virtual displacement of the light source along the axis 9 in the direction of the window 2 according as the light rays enclose a larger angle with the axis 9. The lens 12 has a stepped lens profile consisting of annular zones. Annular transition regions l3, l4, 15 between the zones touch the light rays of which two are denoted by 16 and 17. The lens 12 produces an increasing virtual displacement of the light source at right angles to the axis 9 and in the plane of the drawing, according as the light rays enclose a larger angle with the axis 9. During the exposure the lens 12 is translated, the center describing a circle of a radius of 1.5 mm, by means of a device 18 which is shown diagrammatically.

The invention is not restricted to an exposure device having a correction lens system in which the stepped lens profile is present on the side of the light source. The stepped lens profile may also be present on the other side with the same result. In that case the annular transition zones touch light rays which have already been refracted by the correction lens system. Nor is the invention restricted to a correction lens system having two lenses. If, however, the correction lens system consists of several lenses only the lens having a stepped lens profile is preferably translated. Moreover, the lens which in the embodiment is denoted by 11 need not be rotationally symmetric. By making the lens 11 nonrotationally symmetric, said lens may produce a part of the virtual displacement in the direction of the eccentricity of the beam. This is associated with some virtual displacement at right angles to the direction of the eccentricity of the beam and at right angles to the axial direction, with which the said radial compression of the triplets as a result of the curvature of the display screen can be compensated for.

What is claimed is:

1. An exposure device for projecting aperture patterns from a color selection mask onto a photosensitive layer on a display screen for a color television picture tube comprising, a light source, and optical means positioned between said light source and the photosensitive layer on said display screen for correcting discrepancies between optical and electron optical projections, said optical means comprising at least one lens having a stepped profile of annular zones, adjacent annular zones having annular transition regions therebetween, the surfaces of said regions being parallel to the rays of light in the immediate vicinity.

2. An exposure device as claimed in claim 1 further comprising means for moving said optical means during exposure of said display screen, the center of said optical means describing a closed path during said movement. 

1. An exposure device for projecting aperture patterns from a color selection mask onto a photosensitive layer on a display screen for a color television picture tube comprising, a light source, and optical means positioned between said light source and the photosensitive layer on said display screen for correcting discrepancies between optical and electron optical projections, said optical means comprising at least one lens having a stepped profile of annular zones, adjacent annular zones having annular transition regions therebetween, the surfaces of said regions being parallel to the rays of light in the immediate vicinity.
 2. An exposure device as claimed in claim 1 further comprising means for moving said optical means during exposure of said display screen, the center of said optical means describing a closed path during said movement. 